Re: [9fans] multiple venti servers

["Russ Cox" <rsc@swtch.com>]

wrarena does write every block twice,
but venti will only write the block to the disk once.
so it may take twice as long but it should not
use up twice as much disk space.

russ


On 9/1/06, Tharaneedharan Vilwanathan <vdharani@gmail.com> wrote:
> Hi Russ,
>
> On 8/30/06, Russ Cox <rsc@swtch.com> wrote:
> > > If I am right, one problem with this approach is it becomes tricky
> > > when both venti servers have updates but yet to be sync'ed. And, in
> > > particular, when there are identical data backed up (say, I downloaded
> > > a CD image in two machines and backed up in these two servers), it
> > > leaves two copies. It may be safe but it does leave two copies.
> >
> > You are not right.
> Good to know I am not right! :-) About 2 years back I tried to
> reinstall venti server and noticed some pb with copying old data. In
> response to one of the mails, I had sent this reply:
>
> -----------------------------------------------------------------------------
> From:  vdhar...@infernopark.com - view profile
> Date:  Mon, Aug 23 2004 11:34 am
> Email:   vdhar...@infernopark.com
> Groups:   comp.os.plan9
> ...
>
> i have done similar thing (except fossil) before and i didnt face any
> problem.
>
> but i noticed a couple of things:
>
> - if i invoke 'wrarena' twice, it took twice the space. so i think it
> blindly copies rather than checking for existing data. this raises 2
> questions: is venti still in good shape (since now two blocks for the same
> score may exist)? if i backup up same or similar files in different venti
> servers and later on i merge the venti server data using readarena and
> wrarena, will it leave duplicate data blocks unnecessarily?
>
> - you have to keep the venti score details (vac) seperately all the time.
> else there is no way to get the signatures and venti may become useless
> without it.
> -----------------------------------------------------------------------------
>
> Till now, I have been thinking wrarena wrote a copy of every block
> when I invoked it twice. I think I checked up how much space venti
> used up but I am not sure whether there was actually some other issue.
>
> Thanks
> dharani
>
> >
> > Since venti coalesces writes, it doesn't matter if both servers sync
> > to each other.  You'll waste a little bandwidth resending some writes,
> > but you will not end up with multiple copies of a block on the same
> > venti server.
> >
> > Russ
> >
>
>